There are currently available several types of alarm which may be fitted to vehicles, in particular to cars, to give an audible warning when unauthorised use is made of the vehicle. These devices are normally actuated by opening the vehicle door or by turning the ignition key, and need to be turned on and off by the owner or authorised user of the vehicle. If he forgets to turn the alarm on when he leaves the vehicle, the alarm will not be activated, and, if he forgets to turn the alarm off when he returns to the vehicle, the alarm will sound when he attempts to drive the vehicle away. Even with the alarm on, it is often a simple matter for a thief to disconnect or otherwise render inoperative the alarm system.